The problems of construction of high-thrust hybrid rocket motors have been described extensively in numerous patents dealing with the subject, such German Patent No. DE 44 22 195 of the present applicant, as well as in secondary literature, such as George P. Sutton, Rocket Propulsion Elements, 6th edition, 1992, John Wiley & Sons, pp. 502. One problem is maximizing fuel regression, i.e., the amount of solid fuel which can be made available for the combustion process per time unit. This amount determines the rocket motor's maximum thrust.
Several injection systems for hybrid rockets have been proposed (but none built) which are designed to make the combustion process in the hybrid rocket's motor more closely resemble that of a standard solid fuel motor, where oxidizer and fuel are thoroughly mixed. These injection systems are designed around the idea of conducting the oxidizer through the fuel to the fuel block surface via a multitude of channels, as in U.S. Pat. Nos. 3,177,657, 3,142,152, or German Patent No. DE 44 22 195, or, in some cases, by creating a porous fuel block which can be flooded with oxidizer, as in U.S. Pat. No. 3,203,174. U.S. Pat. No. 3,166,898 suggests injecting the oxidizer into the gap between several cylindrical fuel block segments which are spaced apart.
None of the above have been experimentally verified. Porous designs carry the negative of uncontrolled explosion. The design in U.S. Pat. No. 3,166,898 raises the difficulty of affixing said segments to the motor case, in particular when the fuel block bums out, as well as creating combustion instabilities.
It may be assumed that the injection system proposed in U.S. Pat. No. 3,177,657 and German Patent No. DE 44 22 195 will increase regression rates by creating multiple flame fronts directly on or very near the solid fuel's surface, leading to an increased evaporation of solid fuel. However, until experimentally proven, this remains an assumption. It has been sufficiently demonstrated, however, that fuel regression in hybrids correlates directly with the solid fuel surface area (provided mixing of oxygen and evaporated fuel is thorough and thus combustion efficient). U.S. Pat. No. 3,177,657 and German Patent No. DE 44 22 195 provide for a substantially increased solid fuel surface area. Various port designs are known from solid rockets, such as the star, dendrid, dog bone, etc. A simple port design, however, primarily a cylinder, would be preferable.
A design as described in U.S. Pat. No. 3,177,657 with multiple tubes within the fuel block is rather difficult to construct and to check out, making reusable engines particularly difficult to build. In the present applicant's own experiments, a design as in German Patent No. DE 44 22 195, in which holes are drilled through the fuel block instead of placing tubes in the fuel block (assuming that the fuel-oxidizer combination is not hypergolic), has displayed a significant weakness in the form of oxidizer leakage in between the motor casing and the fuel block, carving out cavitations and ultimately leading to motor case burn-through failure. The experiments have demonstrated that this adverse behavior is hardly preventable, as the fuel itself tends to expand and bend inward as a sequel of heat stress during the combustion process.